Category: Electronics Workbench

Electrical & Electronic gadgets

LF Crystal Tester: Joystick for Oscillator Offset Adjustment

With the joystick button and LM75 temperature sensor running, this chunk of code lets you nudge the nominal DDS oscillator frequency by 1 Hz every 100 ms:

	// Zero-beat oscillator to 10 MHz GPS-locked reference

	printf("Zero beat DDS oscillator against GPS\n");

	TempFreq.fx_64 = CALFREQ;

	u8x8.clearDisplay();
	byte ln = 0;
	u8x8.drawString(0,ln++,"10 MHz Zero Beat");
	u8x8.drawString(0,ln++,"<- Joystick ->");
	u8x8.drawString(0,ln++," Button = set ");

	int32_t OldOffset = OscOffset;

	while (analogRead(PIN_JOYBUTTTON) > 500) {

	int ai = analogRead(PIN_JOY_Y) – 512; // totally ad-hoc axes
	if (ai < -100) {
	OscOffset += 1;
	}
	else if (ai > 100) {
	OscOffset -= 1;
	}

	if (OscOffset != OldOffset) {
	ln = 4;
	sprintf(Buffer,"Offset %8ld",OscOffset);
	u8x8.drawString(0,ln++,Buffer);

	CalcOscillator(OscOffset); // recalculate constants

	TempCount.fx_64 = MultiplyFixedPt(TempFreq,CtPerHz); // recalculate delta phase count

	WriteDDS(TempCount.fx_32.high); // should be 10 MHz out!

	OldOffset = OscOffset;
	}

	Wire.requestFrom(LM75_ADDR,2);
	Temperature.fx_32.high = Wire.read();
	Temperature.fx_32.low = (uint32_t)Wire.read() << 24;
	PrintFixedPtRounded(Buffer,Temperature,3);
	ln = 7;
	u8x8.drawString(0,ln,"DDS Temp");
	u8x8.drawString(16-strlen(Buffer),ln++,Buffer);

	delay(100);
	}

	printf("Oscillator offset: %ld\n",OscOffset);

view raw gistfile1.txt hosted with ❤ by GitHub

While that’s happening, you compare the DDS output to a reference frequency on an oscilloscope:

The top trace (and scope trigger) is the GPS-locked 10 MHz reference, the lower trace is the AD9850 DDS output (not through the MAX4165 buffer amp, because bandwidth). If the frequencies aren’t identical, the DDS trace will crawl left or right with respect to the reference: leftward if the DDS frequency is too high, rightward if it’s too low. If the DDS frequency is way off, then the waveform may scamper or run, with the distinct possibility of aliasing on digital scopes; you have been warned.

The joystick acts as a bidirectional switch, rather than an analog input, with the loop determining the step increment and timing. The ad-hoc axis orientation lets you (well, me) push the joystick against the waveform crawl, which gradually slows down and stops when the offset value makes the DDS output match the reference.

The OLED displays the current status:

The lurid red glow along the bottom is lens flare from the amber LED showing the relay is turned on. The slightly dimmer characters across the middle of the display show how the refresh interacts with the camera shutter at 1/30 s exposure.

N.B.: Normally, you know the DDS clock oscillator frequency with some accuracy. Dividing that value into 2³² (for the AD9850) gives you the delta-phase count / frequency ratio that converts a desired DDS output frequency into the delta-phase value telling the DDS to make it happen.

In this case, I want the output frequency to be exactly 10.000000 MHz, so I’m adjusting the oscillator frequency (nominal 125 MHz + offset), calculating the corresponding count-to-Hz ratio, multiplying the ratio by 10.000000 MHz, stuffing the ensuing count into the DDS, and eyeballing what happens. When the oscillator frequency variable matches the actual oscillator frequency, then the actual output will 10.000000 MHz and the ratio will be correct.

Got it? Took me a while.

Although the intent is to tune for best frequency match and move on, you (well, I) can use this to accumulate a table of frequency offset vs. temperature pairs, from which a (presumably simple) formula can be conjured to render this step unnecessary.

The Arduino source code as a GitHub Gist:

	// 60 kHz crystal tester
	// Ed Nisley – KE4ZNU

	#include <avr/pgmspace.h>
	#include <U8g2lib.h>
	#include <U8x8lib.h>
	#include <Adafruit_MCP4725.h>

	//———————
	// Pin locations

	#define PIN_SYNC 5

	#define PIN_CX_SHORT 6

	#define PIN_DDS_RESET 7
	#define PIN_DDS_LATCH 8

	#define PIN_HEARTBEAT 9

	#define PIN_LOG_AMP A0

	#define PIN_JOYBUTTTON A1
	#define PIN_JOY_Y A2
	#define PIN_JOY_X A3

	// SPI & I2C use hardware support: these pins are predetermined

	#define PIN_SS 10
	#define PIN_MOSI 11
	#define PIN_MISO 12
	#define PIN_SCK 13

	#define PIN_IIC_SDA A4
	#define PIN_IIC_SCL A5

	// IIC Hardware addresses
	// OLED library uses its default address

	#define LM75_ADDR 0x48
	#define SH1106_ADDR 0x70
	#define MCP4725_ADDR 0x60

	// Useful constants

	#define GIGA 1000000000LL
	#define MEGA 1000000LL
	#define KILO 1000LL

	#define ONE_FX (1LL << 32)

	#define CALFREQ (10LL * MEGA * ONE_FX)

	// Structures for 64-bit fixed point numbers
	// Low word = fractional part
	// High word = integer part

	struct ll_fx {
	uint32_t low; // fractional part
	uint32_t high; // integer part
	};

	union ll_u {
	uint64_t fx_64;
	struct ll_fx fx_32;
	};

	// Define semi-constant values

	union ll_u CenterFreq = {(60000 – 4) * ONE_FX}; // center of scan
	//union ll_u CenterFreq = {(32768 – 2) * ONE_FX}; // center of scan

	#define NOMINAL_OSC ((125 * MEGA) * ONE_FX)
	union ll_u Oscillator = {NOMINAL_OSC}; // oscillator frequency
	int32_t OscOffset = -414; // measured offset from NOMINAL_OSC

	uint16_t ScanWidth = 4*2; // width must be an even integer
	uint16_t ScanSettleMS = 2000; // milliseconds of settling time per measurement

	union ll_u ScanStepSize = {ONE_FX / 10}; // 0.1 Hz is smallest practical decimal step
	//union ll_u ScanStepSize = {ONE_FX / 34}; // 0.0291 is smallest possible step

	// Global variables of interest to everyone

	union ll_u ScanFrom, ScanTo; // may be larger than unsigned ints
	union ll_u ScanFreq; // fixed-point frequency scan settings

	union ll_u PeakFreq; // records maximum response point
	union ll_u PeakdB; // and corresponding log amp output

	union ll_u SeriesPeakLow,SeriesPeakHigh; // peak with CX short and CX in circuit

	union ll_u CtPerHz; // will be 2^32 / oscillator
	union ll_u HzPerCt; // will be oscillator / 2^32

	char Buffer[10+1+10+1]; // string buffer for fixed point number conversions

	union ll_u Temperature; // read from LM75A

	// Hardware library variables

	U8X8_SH1106_128X64_NONAME_HW_I2C u8x8(U8X8_PIN_NONE);
	//U8X8_SH1106_128X64_NONAME_4W_HW_SPI u8x8(PIN_DISP_SEL, PIN_DISP_DC , PIN_DISP_RST);
	//U8X8_SH1106_128X64_NONAME_4W_SW_SPI u8x8(PIN_SCK, PIN_MOSI, PIN_DISP_SEL, PIN_DISP_DC , PIN_DISP_RST);

	#define DAC_WR false
	#define DAC_WR_EEP true
	#define DAC_BITS 12
	#define DAC_MAX 0x0fff

	Adafruit_MCP4725 XAxisDAC; // I²C DAC for X axis output
	uint32_t XAxisValue; // DAC parameter uses 32 bits

	union ll_u LogAmpdB; // computed dB value

	#define HEARTBEAT_MS 3000

	unsigned long MillisNow,MillisThen;

	//———–
	// Useful functions

	// Pin twiddling

	void TogglePin(char bitpin) {
	digitalWrite(bitpin,!digitalRead(bitpin)); // toggle the bit based on previous output
	}

	void PulsePin(char bitpin) {
	TogglePin(bitpin);
	TogglePin(bitpin);
	}

	void WaitButtonDown() {
	word ai;

	do {
	ai = analogRead(PIN_JOYBUTTTON);
	} while (ai > 500);
	}

	void WaitButtonUp() {
	word ai;

	do {
	ai = analogRead(PIN_JOYBUTTTON);
	} while (ai < 500);
	}

	// Hardware-assisted SPI I/O

	void EnableSPI(void) {
	digitalWrite(PIN_SS,HIGH); // set SPI into Master mode
	SPCR \|= 1 << SPE;
	}

	void DisableSPI(void) {
	SPCR &= ~(1 << SPE);
	}

	void WaitSPIF(void) {
	while (! (SPSR & (1 << SPIF))) {
	// TogglePin(PIN_HEARTBEAT);
	// TogglePin(PIN_HEARTBEAT);
	continue;
	}
	}

	byte SendRecSPI(byte Dbyte) { // send one byte, get another in exchange
	SPDR = Dbyte;
	WaitSPIF();

	return SPDR; // SPIF will be cleared
	}

	//————–
	// DDS module

	void EnableDDS(void) {

	digitalWrite(PIN_DDS_LATCH,LOW); // ensure proper startup

	digitalWrite(PIN_DDS_RESET,HIGH); // minimum reset pulse 40 ns, not a problem
	digitalWrite(PIN_DDS_RESET,LOW);
	delayMicroseconds(1); // max latency 100 ns, not a problem

	DisableSPI(); // allow manual control of outputs
	digitalWrite(PIN_SCK,LOW); // ensure clean SCK pulse
	PulsePin(PIN_SCK); // … to latch hardwired config bits
	PulsePin(PIN_DDS_LATCH); // load hardwired config bits = begin serial mode

	EnableSPI(); // turn on hardware SPI controls
	SendRecSPI(0x00); // shift in serial config bits
	PulsePin(PIN_DDS_LATCH); // load serial config bits

	}

	// Write delta phase count to DDS
	// This comes from the integer part of a 64-bit scaled value

	void WriteDDS(uint32_t DeltaPhase) {

	SendRecSPI((byte)DeltaPhase); // low-order byte first
	SendRecSPI((byte)(DeltaPhase >> 8));
	SendRecSPI((byte)(DeltaPhase >> 16));
	SendRecSPI((byte)(DeltaPhase >> 24));

	SendRecSPI(0x00); // 5 MSBs = phase = 0, 3 LSBs must be zero

	PulsePin(PIN_DDS_LATCH); // write data to DDS
	}

	//————–
	// Log amp module

	#define LOG_AMP_SAMPLES 10
	#define LOG_AMP_DELAYMS 10

	uint64_t ReadLogAmp() {

	union ll_u LogAmpRaw;

	LogAmpRaw.fx_64 = 0;
	for (byte i=0; i<LOG_AMP_SAMPLES; i++) {
	LogAmpRaw.fx_32.high += analogRead(PIN_LOG_AMP);
	delay(LOG_AMP_DELAYMS);
	}

	LogAmpRaw.fx_64 /= LOG_AMP_SAMPLES; // figure average from totally ad-hoc number of samples

	LogAmpRaw.fx_64 *= 5; // convert from ADC counts to voltage
	LogAmpRaw.fx_64 /= 1024;

	LogAmpRaw.fx_64 /= 24; // convert from voltage to dBV at 24 mV/dBV
	LogAmpRaw.fx_64 *= 1000;

	return LogAmpRaw.fx_64;
	}

	//———–
	// Scan DDS and record response

	void ScanCrystal() {

	byte ln;
	union ll_u Temp, TestFreq, TestCount;

	XAxisValue = 0;
	PeakdB.fx_64 = 0;

	printf("CX: %s\n",digitalRead(PIN_CX_SHORT) ? "short" : "enable");

	for (ScanFreq = ScanFrom;
	ScanFreq.fx_64 < (ScanTo.fx_64 + ScanStepSize.fx_64 / 2);
	ScanFreq.fx_64 += ScanStepSize.fx_64) {

	digitalWrite(PIN_SYNC,HIGH);

	TestCount.fx_64 = MultiplyFixedPt(ScanFreq,CtPerHz); // compute DDS delta phase
	TestCount.fx_32.low = 0; // truncate count to integer
	TestFreq.fx_64 = MultiplyFixedPt(TestCount,HzPerCt); // compute actual frequency

	Temp.fx_64 = (DAC_MAX * (ScanFreq.fx_64 – ScanFrom.fx_64)); // figure X as fraction
	Temp.fx_64 /= ScanWidth;
	XAxisValue = Temp.fx_32.high;

	digitalWrite(PIN_HEARTBEAT,HIGH);
	WriteDDS(TestCount.fx_32.high); // set DDS to new frequency
	XAxisDAC.setVoltage(XAxisValue,DAC_WR); // and set X axis to match
	digitalWrite(PIN_SYNC,LOW);

	if (ScanFreq.fx_64 == ScanFrom.fx_64) {
	delay(3*ScanSettleMS); // very long settling time
	}
	else {
	delay(ScanSettleMS); // small steps are faster
	}

	LogAmpdB.fx_64 = ReadLogAmp(); // fetch avg value

	if (LogAmpdB.fx_64 > PeakdB.fx_64) { // hit a new high?
	PeakFreq = TestFreq; // save actual frequency
	PeakdB = LogAmpdB;

	ln = digitalRead(PIN_CX_SHORT) ? 4 : 5; // CX selects row
	PrintFixedPtRounded(Buffer,TestFreq,2); // display actual peak
	u8x8.drawString(0,ln,Buffer);

	PrintFixedPtRounded(Buffer,LogAmpdB,1); // tack on response
	u8x8.drawString(16-strlen(Buffer),ln,Buffer);
	}

	ln = 0;
	PrintFixedPtRounded(Buffer,TestFreq,2); // display current frequency
	u8x8.draw2x2String(0,ln++,Buffer);
	ln++; // double-high characters
	printf("%9s ",Buffer); // log to serial port

	PrintFixedPtRounded(Buffer,LogAmpdB,1); // display response
	u8x8.drawString(0,ln,"dBV ");
	u8x8.drawString(16-strlen(Buffer),ln++,Buffer);
	printf(", %6s\n",Buffer); // and log it

	}

	}

	//———–
	// Round scaled fixed point to specific number of decimal places: 0 through 8
	// You should display the value with only Decimals characters beyond the point
	// Must calculate rounding value as separate variable to avoid mystery error

	uint64_t RoundFixedPt(union ll_u TheNumber,unsigned Decimals) {
	union ll_u Rnd;

	Rnd.fx_64 = (ONE_FX >> 1) / (pow(10LL,Decimals)); // that's 0.5 / number of places
	TheNumber.fx_64 = TheNumber.fx_64 + Rnd.fx_64;

	return TheNumber.fx_64;
	}


	//———–
	// Multiply two unsigned scaled fixed point numbers without overflowing a 64 bit value
	// Perforce, the product of the two integer parts mut be < 2^32

	uint64_t MultiplyFixedPt(union ll_u Mcand, union ll_u Mplier) {
	union ll_u Result;

	Result.fx_64 = ((uint64_t)Mcand.fx_32.high * (uint64_t)Mplier.fx_32.high) << 32; // integer parts (clear fract)
	Result.fx_64 += ((uint64_t)Mcand.fx_32.low * (uint64_t)Mplier.fx_32.low) >> 32; // fraction parts (always < 1)
	Result.fx_64 += (uint64_t)Mcand.fx_32.high * (uint64_t)Mplier.fx_32.low; // cross products
	Result.fx_64 += (uint64_t)Mcand.fx_32.low * (uint64_t)Mplier.fx_32.high;

	return Result.fx_64;
	}


	//———–
	// Long long print-to-buffer helpers
	// Assumes little-Endian layout

	void PrintHexLL(char *pBuffer,union ll_u FixedPt) {
	sprintf(pBuffer,"%08lx %08lx",FixedPt.fx_32.high,FixedPt.fx_32.low);
	}

	// converts all 9 decimal digits of fraction, which should suffice

	void PrintFractionLL(char *pBuffer,union ll_u FixedPt) {
	union ll_u Fraction;

	Fraction.fx_64 = FixedPt.fx_32.low; // copy 32 fraction bits, high order = 0
	Fraction.fx_64 *= GIGA; // times 10^9 for conversion
	Fraction.fx_64 >>= 32; // align integer part in low long
	sprintf(pBuffer,"%09lu",Fraction.fx_32.low); // convert low long to decimal
	}

	void PrintIntegerLL(char *pBuffer,union ll_u FixedPt) {
	sprintf(pBuffer,"%lu",FixedPt.fx_32.high);
	}

	void PrintFixedPt(char *pBuffer,union ll_u FixedPt) {
	PrintIntegerLL(pBuffer,FixedPt); // do the integer part
	pBuffer += strlen(pBuffer); // aim pointer beyond integer
	*pBuffer++ = '.'; // drop in the decimal point, tick pointer
	PrintFractionLL(pBuffer,FixedPt);
	}

	void PrintFixedPtRounded(char *pBuffer,union ll_u FixedPt,unsigned Decimals) {
	char *pDecPt;

	FixedPt.fx_64 = RoundFixedPt(FixedPt,Decimals);

	PrintIntegerLL(pBuffer,FixedPt); // do the integer part
	pBuffer += strlen(pBuffer); // aim pointer beyond integer

	pDecPt = pBuffer; // save the point location
	*pBuffer++ = '.'; // drop in the decimal point, tick pointer

	PrintFractionLL(pBuffer,FixedPt); // do the fraction

	if (Decimals == 0)
	*pDecPt = 0; // 0 places means discard the decimal point
	else
	*(pDecPt + Decimals + 1) = 0; // truncate string to leave . and Decimals chars

	}

	//———–
	// Calculate useful "constants" from oscillator info
	// Offset is integer Hz, because 0.1 ppm = 1 Hz at 10 MHz is as close as we can measure

	void CalcOscillator(int32_t Offset) {

	Oscillator.fx_64 = NOMINAL_OSC + ((int64_t)Offset << 32);

	HzPerCt.fx_32.low = Oscillator.fx_32.high; // divide oscillator by 2^32 with simple shifting
	HzPerCt.fx_32.high = 0;

	CtPerHz.fx_64 = -1; // Compute (2^32 – 1) / oscillator
	CtPerHz.fx_64 /= (uint64_t)Oscillator.fx_32.high; // remove 2^32 scale factor from divisor

	}

	//– Helper routine for printf()

	int s_putc(char c, FILE *t) {
	Serial.write(c);
	}

	//———–

	void setup () {

	union ll_u TempFreq,TempCount;

	pinMode(PIN_HEARTBEAT,OUTPUT);
	digitalWrite(PIN_HEARTBEAT,LOW); // show we got here

	pinMode(PIN_SYNC,OUTPUT);
	digitalWrite(PIN_SYNC,LOW);

	Serial.begin (115200);
	fdevopen(&s_putc,0); // set up serial output for printf()

	Serial.println (F("60 kHz Crystal Tester"));
	Serial.println (F("Ed Nisley – KE4ZNU – June 2017\n"));

	// DDS module controls

	pinMode(PIN_DDS_LATCH,OUTPUT);
	digitalWrite(PIN_DDS_LATCH,LOW);
	pinMode(PIN_DDS_RESET,OUTPUT);
	digitalWrite(PIN_DDS_RESET,HIGH);

	// Light up the display

	Serial.println("Initialize OLED");
	u8x8.begin();
	u8x8.setFont(u8x8_font_artossans8_r);
	// u8x8.setPowerSave(0);

	u8x8.setFont(u8x8_font_pxplusibmcga_f);
	u8x8.draw2x2String(0,0,"XtalTest");
	u8x8.drawString(0,3,"Ed Nisley");
	u8x8.drawString(0,4," KE4ZNU");
	u8x8.drawString(0,6,"June 2017");

	// configure SPI hardware

	pinMode(PIN_SS,OUTPUT); // set up manual controls
	digitalWrite(PIN_SS,HIGH);
	pinMode(PIN_SCK,OUTPUT);
	digitalWrite(PIN_SCK,LOW);
	pinMode(PIN_MOSI,OUTPUT);
	digitalWrite(PIN_MOSI,LOW);

	pinMode(PIN_MISO,INPUT_PULLUP);

	SPCR = B00110000; // Auto SPI: no int, disabled, LSB first, master, + edge, leading, f/4
	SPSR = B00000000; // not double data rate

	TogglePin(PIN_HEARTBEAT); // show we got here

	// Set up X axis DAC output

	XAxisDAC.begin(MCP4725_ADDR); // start up MCP4725 DAC at Sparkfun address
	// XAxisDAC.setVoltage(0,DAC_WR_EEP); // do this once per DAC to set power-on at 0 V
	XAxisDAC.setVoltage(0,DAC_WR); // force 0 V after a reset without a power cycle

	// LM75A temperature sensor requires no setup!

	// External capacitor in test fixture

	pinMode(PIN_CX_SHORT,OUTPUT);
	digitalWrite(PIN_CX_SHORT,HIGH); // short = remove external cap

	// Scan limits and suchlike

	ScanFrom.fx_64 = CenterFreq.fx_64 – (ONE_FX * ScanWidth/2);
	ScanTo.fx_64 = CenterFreq.fx_64 + (ONE_FX * ScanWidth/2);

	PrintFixedPtRounded(Buffer,CenterFreq,1);
	printf("Center freq: %s Hz\n",Buffer);
	printf("Settling time: %d ms\n",ScanSettleMS);

	// Wake up and load the DDS

	CalcOscillator(OscOffset); // use default oscillator frequency

	Serial.print("\nStarting DDS: ");
	TempFreq.fx_64 = CALFREQ;
	TempCount.fx_64 = MultiplyFixedPt(TempFreq,CtPerHz);
	// PrintHexLL(Buffer,TempCount);
	// printf(" Count: %s ",Buffer);

	EnableDDS();

	WriteDDS(TempCount.fx_32.high);
	Serial.println("running\n");

	// Zero-beat oscillator to 10 MHz GPS-locked reference

	printf("Zero beat DDS oscillator against GPS\n");

	TempFreq.fx_64 = CALFREQ;

	u8x8.clearDisplay();
	byte ln = 0;
	u8x8.drawString(0,ln++,"10 MHz Zero Beat");
	u8x8.drawString(0,ln++,"<- Joystick ->");
	u8x8.drawString(0,ln++," Button = set ");

	int32_t OldOffset = OscOffset;

	while (analogRead(PIN_JOYBUTTTON) > 500) {

	int ai = analogRead(PIN_JOY_Y) – 512; // totally ad-hoc axes
	if (ai < -100) {
	OscOffset += 1;
	}
	else if (ai > 100) {
	OscOffset -= 1;
	}

	if (OscOffset != OldOffset) {
	ln = 4;
	sprintf(Buffer,"Offset %8ld",OscOffset);
	u8x8.drawString(0,ln++,Buffer);

	CalcOscillator(OscOffset); // recalculate constants

	TempCount.fx_64 = MultiplyFixedPt(TempFreq,CtPerHz); // recalculate delta phase count

	WriteDDS(TempCount.fx_32.high); // should be 10 MHz out!

	OldOffset = OscOffset;
	}

	Wire.requestFrom(LM75_ADDR,2);
	Temperature.fx_32.high = Wire.read();
	Temperature.fx_32.low = (uint32_t)Wire.read() << 24;
	PrintFixedPtRounded(Buffer,Temperature,3);
	ln = 7;
	u8x8.drawString(0,ln,"DDS Temp");
	u8x8.drawString(16-strlen(Buffer),ln++,Buffer);

	delay(100);
	}

	printf("Oscillator offset: %ld\n",OscOffset);

	u8x8.clearDisplay();

	Serial.println("\nStartup done\n");

	MillisThen = millis();
	}

	//———–

	void loop () {

	byte ln;
	union ll_u Temp;

	u8x8.setPowerSave(0);
	u8x8.clearDisplay();
	ln = 0;
	u8x8.draw2x2String(0,2*ln++,"Press");
	u8x8.draw2x2String(0,2*ln++,"Button");
	u8x8.draw2x2String(0,2*ln++,"To Start");
	u8x8.draw2x2String(0,2*ln++,"Test");
	printf("Waiting for button press: ");

	WaitButtonDown();

	printf("\n");

	u8x8.clearDisplay();
	// u8x8.setPowerSave(1);

	// Report temperature

	Wire.requestFrom(LM75_ADDR,2);
	Temperature.fx_32.high = Wire.read();
	Temperature.fx_32.low = (uint32_t)Wire.read() << 24;
	PrintFixedPtRounded(Buffer,Temperature,3);
	printf("Oscillator temperature: %s C\n",Buffer);
	ln = 3;
	u8x8.drawString(0,ln,"DDS Temp");
	u8x8.drawString(16-strlen(Buffer),ln,Buffer);

	// First scan: CX shorted

	digitalWrite(PIN_CX_SHORT,HIGH);
	delay(10);

	ScanCrystal();

	SeriesPeakLow = PeakFreq;

	PrintFixedPtRounded(Buffer,PeakFreq,2); // report peak freq
	printf("\nPeak: %s Hz",Buffer);

	PrintFixedPtRounded(Buffer,PeakdB,1); // tack on response
	printf(" %s dbV\n",Buffer);

	// Second scan: CX in circuit

	digitalWrite(PIN_CX_SHORT,LOW);
	delay(10);

	ScanFrom.fx_64 = SeriesPeakLow.fx_64 – (2 * ONE_FX); // tighten scan limits
	ScanFrom.fx_32.low = 0;
	ScanTo.fx_64 = SeriesPeakLow.fx_64 + (4 * ONE_FX);
	ScanTo.fx_32.low = 0;

	ScanCrystal();

	SeriesPeakHigh = PeakFreq;

	PrintFixedPtRounded(Buffer,PeakFreq,2); // report peak freq
	printf("\nPeak: %s Hz",Buffer);

	PrintFixedPtRounded(Buffer,PeakdB,1); // tack on response
	printf(" %s dbV\n",Buffer);

	ln = 0;
	u8x8.draw2x2String(0,ln," -Done- ");
	ln +=2;
	u8x8.clearLine(ln);

	ln = 6;
	Temp.fx_64 = SeriesPeakHigh.fx_64 – SeriesPeakLow.fx_64;
	PrintFixedPtRounded(Buffer,Temp,2);
	printf("Delta frequency: %s\n",Buffer);
	u8x8.drawString(0,ln,"Delta freq");
	u8x8.drawString(16-strlen(Buffer),ln,Buffer);

	ln = 7;
	u8x8.drawString(0,ln,"Press button …");

	u8x8.setPowerSave(0);

	WaitButtonDown();

	WaitButtonUp();


	}

view raw CrystalTester.ino hosted with ❤ by GitHub

2017-07-03

Monthly Science: Cheap WS2812 LED Failures

The two knockoff Neopixel test fixtures went dark while their USB charger accompanied me on a trip, so they spent a few days at ambient basement conditions. When I plugged them back into the charger, pretty much the entire array lit up in pinball panic mode:

WS2812 LED – test fixture multiple failures

Turns out three more WS2812 chips failed in quick succession. I’ve hotwired around the deaders (output disconnected, next chip input in parallel) and, as with the other zombies, they sometimes work and sometimes flicker. That’s five failures in 28 LEDs over four months, a bit under 3000 operating hours.

For lack of a better explanation: the cool chips pulled relatively moist air through their failed silicone encapsulation, quietly rotted out in the dark, then failed when reheated. After they spend enough time flailing around, the more-or-less normal operating temperatures drives out the moisture and they (sometimes) resume working.

Remember, all of them passed the Josh Sharpie Test, so you can’t identify weak ones ahead of time.

2017-07-01
Arduino Joystick: Button Pullup FAIL

I wired a resistive joystick to the knockoff Nano controlling the crystal tester and connected the button to an analog input because I have a lot of those left over and why not. Unfortunately, the ADC returned a sequence of random-ish numbers indicating the button didn’t have a pullup to +5 V.

One might be forgiven for assuming the pads marked R5 would hold such a pullup resistor, had the joystick not been relentlessly cost-reduced:

Keyes resistive joystick – R5 location

One would, of course, be completely wrong.

Having been around this block several times, I measured the pad-to-pin resistances and found R5 firmly affixed to the GND and +5V pins, with the SW (a.k.a. button) pin floating free. Pressing the joystick hat closes the switch next to R5, thereby connecting the SW pin to GND.

Baffles me. Maybe a fresh intern did the PCB layout and just misplaced the resistor?

So I soldered an ordinary resistor (*) between the +5 V and SW pins:

Keyes resistive joystick – button pullup

Now it works just as it should.

(*) For long-lost reasons, I have a zillion 12.4 kΩ 1% resistors appearing in place of simple 10 kΩ resistors.

2017-06-29
LM75A vs. SOIC Adapter: Mirror Imaging

After hairballing an LM75A I²C temperature sensor to verify at least one of the eBay lot worked, a bag of SOIC-to-DIP space transformers arrived, so I soldered up another LM75A:

LM75A on DIP8 adapter – top

The SOIC chip pattern sits at right angles to the DIP pins, which took some getting used to.

The slightly defocused wire connecting pin 4 (on the IC) to pins 5, 6, and 7 (on the PCB) selects address 0x48.

So I flipped it over, soldered four wires (+5 V, GND, SDA, SCL) to the numbered pins on bottom of the board, made up a little header for the other end, wired a socket strip on the crystal tester board, plugged it in, and … nothing worked.

Turns out that the other side of the board carries a TSSOP pattern, which I’d neatly masked off with a snippet of Kapton tape, surrounded by eight numbered pins. Of course, those pin numbers correspond to the TSSOP pattern facing you, so they’re mirror-imaged for the SOIC pattern on the other side.

Soooo, the proper wiring for the SOIC pattern as seen from the TSSOP side has the pin numbers exactly bass-ackwards:

LM75A on DIP8 adapter – bottom

The insulation looked a lot better the first time I soldered the wires to the PCB. Honest.

Anyhow, when correctly wired, the LM75A worked as it should:

LM75A Temperature Sensor – installed

It’s snuggled chip-down against the top of the 125 MHz oscillator can, with a dab of heatsink compound improving their thermal bond and a yellow cable tie around the foam holding them together. The socket header is wired pin-for-pin to the DAC I²C socket directly above it.

The OLED temperature display shows 28.250 °C, because the oscillator just started up in a cool basement. It’ll eventually settle around 39-ish °C, where its output should be pretty close to the 125 MHz – 344 Hz value hardcoded into the source.

Oh, that’s a 3 mm amber LED next to the relay can: much less glaring than the white LED, no matter what it looks like here.

2017-06-27
LF Crystal Tester: Bring the Noise!

The OLED display refresh contributes 100 Hz noise pulses to the low-level sine wave from the crystal test fixture:

OLED Enabled – 100 Hz display refresh

Disabling the display by activating its powersave option reveals 60 Hz pulses from the USB port on the Arduino Nano:

OLED Powersave – 60 Hz USB Ground Loop

Unplugging the USB cable, leaving just the +5 VDC power supply and coax cable to the oscilloscope, solves most of the problem:

OLED Powersave – USB unplugged

A closer look shows some (relatively) low frequency noise remains in full effect:

OLED Powersave – USB unplugged – detail

Disabling the display while measuring the crystal seems sensible, although, to avoid surprises, a pushbutton should start the process. Unplugging the USB port puts a real crimp in the data collection, although that’s probably survivable with a USB isolator, one of which is on the way around the planet.

The remaining low-level chop requires more thought. Somewhat to my surprise, holding the Arduino Reset button down doesn’t change much of anything, so it’s not a firmware thing.

Those 10 µF coupling caps gotta go.

With the OLED dark and the USB carrying data:

Spectrum – OLED Powersave – USB in

Compare that to the first pass:

Spectrum-60

Tamping down the noise seems to reduce the overall amplitude variation, but it also makes the capacitor-in and capacitor-out curves more consistent. There may be other things going on that I haven’t accounted for.

The peak frequencies differ by 0.2 Hz, which is probably due to a few degrees of temperature difference. Obviously, it’s badly in need of a temperature calibration & correction.

2017-06-23
LF Crystal Tester: LM75 Temperature Sensor
A strip of NXP (née Philips plus Freescale, including the part of Motorola that didn’t become ON) LM75A I²C temperature sensors arrived from beyond the horizon. To see if they worked, I soldered thin wires directly to the SO-8 pins, entombed it in Kapton tape to prevent spitzensparken, and jammed it under the foam insulation atop the AD9850 DDS module:

AD9850 DDS module with LM75A Temperature Sensor

This turns out to be easier than screwing around with thermistors, because the chip reports the temperature directly in Celcius with ⅛ °C resolution. Classic LM75 chips from National (now absorbed by TI) had ½ °C resolution, but the datasheet shows the bits have an easily extensible format:

LM75A Temperature Data Format

Huh. Fixed-point data, split neatly on a byte boundary. Who’d’a thunk it?

There’s a standard Arduino library using, naturally enough, floating point numbers, but I already have big fixed point numbers lying around and, with the I²C hardware up & running from the X axis DAC and OLED display, this was straightforward:
```
Wire.requestFrom(LM75_ADDR,2);
Temp.fx_32.high = Wire.read();
Temp.fx_32.low = (uint32_t)Wire.read() << 24;
PrintFixedPtRounded(Buffer,Temp,3);
u8x8.drawString(0,ln,"DDS C          ");
u8x8.drawString(16-strlen(Buffer),ln,Buffer);
printf(",%s",Buffer);
ln += 1;
```
The next-to-last line squirts the temperature through the serial port to make those nice plots.

Casually ignoring all I²C bus error conditions will eventually lead to heartache and confusion. In particular, the Basement Laboratory temperature must never fall below 0 °C, because I just plunk the two’s-complement temperature data into an unsigned fixed point number.

Which produces the next-to-bottom line:

DDS OLED with LM75 temperature

Alas, the u8x8 font doesn’t include a degree symbol.

Given sufficient motivation, I can now calibrate the DDS output against the GPS-locked 10 MHz standard to get a (most likely) linear equation for the oscillator frequency offset as a function of temperature. The DDS module includes a comparator to square up its sine wave, so an XOR phase detector or something based on filtering the output of an analog switch might be feasible.
2017-06-22
LF Crystal Tester: Fixture Output Amp

The crystal test fixture and amp huddle in front of the OLED display:

Crystal Tester – First Light

The schematic:

Test fixture – Relay – MAX4255 amp – schematic

The 22 pF cap now sits across the relay’s NO contacts, so as to simplify measuring the total in-circuit capacitance. The LED turns on when the relay shorts out the capacitor, which has a 50% probability of making more sense.

The quartz tuning fork resonators have an ESR around 20 or 30 kΩ, so the off-resonance output should be down something like -60 dB = 20 log (24 / 24×10³) from the 150 mV input: 200 µV (-ish). It’s actually around 1 mV, suggesting plenty of blowby through the baling-wire connections hidden under that neat top surface. I think that’s why the whole setup shows only about 8 dB of dynamic range; more attention to detail may be in order, although the peaks probably won’t move all that much.

Anyhow, even though the AD8310 log amp module should be able to handle such a tiny signal, the MAX4255 amp provides 40 dB of gain (OK, just 39.8 dB) and rolls off the high end at 220 kHz as a side benefit of its 22 MHz GBW.

There’s way too much low frequency rumble at the amp output:

100 Hz noise at MA4255 output

What look like grass is actually the 60 kHz resonator output: those big lumps & bumps are noise from this-and-that. The repetitive peaks and dents exactly 10 ms apart (the cursors span four of ’em) felt a lot like OLED refresh cycles and, indeed, went away when I yanked the display out. Pulling the USB connection eliminates another tremendous heap o’ noise, so there’s likely a ground loop (-ish) thing going on, too. This may call for a USB optical isolator, its commercial equivalent, or more eBay offerings. Getting rid of that junk may improve the dynamic range enough to keep me from doing anything drastic.

The AD8310 log amp input now has decent coupling caps, so it’s not seeing the VCC/2 bias, and I removed that kludged-in 50 Ω terminating resistor to present its full 1.1 kΩ input resistance to the op amp.

2017-06-21